Thermal transfer printing receiver sheet

ABSTRACT

A thermal transfer printing receiver sheet comprises a substrate having on one side a dye receiving layer, wherein the substrate comprises a porous material, preferably a porous plastics material having a network of interconnecting pores communicating throughout the substrate and wherein dye from an image printed in the dye receiving layer is prevented from permeating into the substrate by a sub-layer interposed between the substrate and the receiving layer.

This invention relates to a thermal transfer printing receiver sheet,and in particular to a receiver sheet having a porous substrate.

Thermal transfer printing is a printing process in which a dye is causedby thermal stimuli to transfer from a dye sheet to a receiver sheet bydiffusion and/or sublimation. In such processes the dye sheet andreceiver sheet are placed in intimate contact, the thermal stimuli areapplied to pre-determined areas of the dye sheet and the dye isselectively transferred to the receiver to form the desired image. Thedye sheet and receiver sheet are then separated.

Receiver sheets conventionally comprise a substrate with a dye receivingsurface on one side into which a dye is thermally transferable andretainable. The dye-receiving surface may be provided by one side of thesubstrate, however, receiver sheets typically comprise a substratesupporting a receiving layer which layer presents a dye-receivingsurface. The receiving layer typically comprises a dye-receptivepolymer, a cross-linking agent and a release system.

Substrates conventionally employed in thermal transfer printing includethermoplastic films, for example polyethylene terephthalate, andlaminated paper substrates.

Receiver sheets, for example in the form of a card, have found wideusage in security applications, for example credit cards, charge cards,identification cards, driving licences and passports.

Where the receiver sheet comprises a solid plastics substrate problemswith security may be encountered where, on attempting to replace a layeron the sheet by delaminating the layers within the recording sheet,fracture tends to occur at the boundary between adjoining layers ratherthan through the substrate itself. Consequently there may not besignificant evidence that the receiver sheet has been tampered with andthus a significant security risk my be presented. Laminated papersubstrates also have a drawback in that they have relatively poordurability when exposed to solvents or water as may occur in for exampleflush cut card applications, that is where the edge of a card is removedduring the production process and presents an exposed cross-section ofthe layers in the card.

U.S. Pat. No. 4,861,644 discloses the use of a micro-porous material asa printing substrate for printing inks. There is however no disclosureof such substrates being suitable for use with thermally transferabledyes in thermal transfer printing.

We have now found that drawbacks associated with prior art receiversheets may be reduced by providing substrate comprising a porousplastics material.

According to a first aspect of the invention, there is provided athermal transfer printing receiver sheet comprising a substrate havingon one side thereof a dye receiving layer wherein the substratecomprises a porous plastics material having a network of interconnectingpores communicating throughout the substrate.

Such a substrate provides excellent durability and resistance tosolvents or water as compared with laminated paper substrates. Asignificant practical benefit of these advantages is that the wear,ageing and handling characteristics of the receiver sheet aresignificantly improved.

Furthermore, the porous structure of the substrate employed in thepresent invention reduces the possibility of fracture occurring betweenadjacent layers of the receiver sheet in the event that delamination isattempted as the substrate itself may fracture internally beforedelamination occurs. Thus a greater degree of security is provided dueto the reduced possibility of delamination of the layers of the receiversheet, and also due to the evidence of tampering with the receiver sheetprovided by the fracture substrate of the receiver sheet.

The substrate is preferably a single layer rather than a multi-layerlaminate although a laminate may be employed as the substrate ifdesired. The preferred single layer substrate may comprise one porousplastics material or, if desired, a plurality of porous plasticsmaterials which are suitably mixed or blended, preferably to provide asubstantially homogenous polymer blend such that a single layersubstrate is provided.

Suitable porous plastics materials for use in the present inventioninclude polyolefins, for example polyethylene, polypropylene andpolybutene.

The substrate may also comprise other components such as otherthermoplastic polymers, for example acrylic acid/polyethylenecopolymers, and fillers, for example silica.

Preferably, the pores have a maximum dimension of 0.01 to 20 microns,more preferably 0.1 to 10 microns, especially 0.3 to 5 microns. Suitablythe pores are generally spherical cavities but, if desired, may befissures in which the length of the pore is significantly greater thanthe width thereof. The pore shape and size may be tailored as desired bystretching the substrate either uniaxially or biaxially.

Suitably, the porous plastics material has a void volume of at least50%, that is, for any given volume of the plastic material, the poresrepresent at least 50% of that volume. Without a significant voidvolume, the substrate is less likely to fracture upon attempteddelamination of the receiver sheet. Preferably, the void volume not inexcess of 80% as a greater void volume may lead to structuralinstability of the substrate which may thus lack suitable durability. Aparticularly preferred void volume is in the range 60 to 70% whichsuitably provides a balance of durability and protection againstdelamination without fracture of the substrate.

The substrate is suitably produced by a method, as for example disclosedin U.S. Pat. No. 4,861,644, which involves extruding a mixturecomprising a plastics material, optionally with other components, whichis to form the substrate and a processing material and then forming asheet by passing the said materials through a sheeting die. Suitably,the processing material is then removed from the sheet, for example bysolvent extraction and subsequent removal of any residues of the saidsolvent, to form a substrate comprising a porous plastics material.

Although a porous substrate has the various advantages listed above, italso has one serious disadvantage in that it is permeable to the dyesforming the printed image in the receiver layer, in particular magentadye. Thus, when a receiver layer is placed directly on a poroussubstrate, the image dye has a tendency to diffuse out of the receiverlayer into the substrate leading to a less dense image and, because ofthe preferential permeability to magneta dye, a colour imbalance in theimage. In extreme cases, the dye can permeate completely through thesubstrate causing discolouration of the rear surface.

Under certain circumstances, receiver sheets can be subjected toextremely high ambient temperature, for example when exposed to sunlightin a vehicle an ambient temperature up to 60° C., and possibly as highas 80° C., may be reached and moreover, there is a specific requirementby the International Civil Aviation Agency that machine readablepassports can withstand being stored at such temperatures.Unfortunately, the diffusion effect increases with temperature and atthese higher temperatures there can be a significant deterioration ofthe image.

It is a further object of this invention to reduce the diffusion effectthus enabling the use of porous substrates in identity card typesituations where high ambient temperatures are possible.

According to a further aspect of the invention, there is provided athermal transfer printing receiver sheet comprising a substrate havingon one side a dye receiving layer, wherein the substrate is a porousmaterial and a sub-layer is interposed between the substrate and the dyereceiving layer, the sub-layer being such as to provide a dyepermeability value at 60° C. of less than 20%.

The term dye permeability value is herein defined as being thepercentage reduction in the measured Optical Density of the printedimage at a specified temperature.

Preferably the sub-layer is such as to provide a dye permeability valueat 80° C. of less than 20%.

Whilst the substrate is preferably a plastics material as disclosed inU.S. Pat. No. 4,861,644, it may alternatively be a resin bonded papersuch as type E86016 supplied by Felix Schoeller, or ordinary plainpaper.

As mentioned previously, it is advantageous if, in the event ofdelamination being attempted, fracture occurs in the substrate itselfrather than at the interface between the layers. Hence it is desirablethat the sub-layer has good adhesion and according to a preferredfeature of the invention, the bond strength to the substrate is at least10 N/cm.

The porosity of the surface of the substrate means that there is atendency for coatings suitable for solid or foamed substrates to soakinto the surface producing a grainy finish which can be reflected in theimage appearance. Hence it is desirable that the sub-coat be such as toprovide good filling and smoothing of the surface and to this end afurther preferred feature of the invention provides for the sub-layer tohave a solids content when applied of at least 20%.

The sub-layer provides for excellent bonding between the substrate andthe dye-receiving layer as a consequence of which any fracture whichoccurs in the receiver sheet due to attempted delamination is lesslikely to occur along a plane between adjacent layers in the sheet andsuitably occurs within a layer, preferably the substrate, thus providingevidence of tampering.

Preferably, the sub-layer is relatively soft so that any fracture in thereceiver sheet is more likely to occur in the substrate than in thesub-layer.

The sub-layer is preferably substantially resistant to solventsconventionally employed in fill coating processes. Further, thesub-layer is preferably also impermeable to the materials in the dyereceiving layer and suitably presents a barrier between the substrateand the dye receiving layer to reduce the possibility of absorption ofmaterials by the substrate which may cause variation in the compositionor thickness of the dye-receiving layer or leave areas of the substrateexposed at the surface of the receiver sheet.

Suitably, the sub-layer comprises an acrylic acid/vinyl acetatecopolymer, an acrylic acid/vinylidene chloride copolymer, or a polyvinyl alcohol.

The bond strength may be improved if required by the addition of, forexample a sulphonated polyester, although at the expense of a slightincrease in the permeability value.

The receiver sheet may comprise a back coat on the opposite side of thesubstrate to the dye-receiving surface to impart desirable propertiesfor example, to improve handling characteristics and to aid adhesion ofa protective cover sheet to the sheet.

Suitably a receiver sheet according to the present invention islaminated with a cover sheet on both sides following imaging to provideprotection for the images on the sheet. The cover sheet may be the sameor different on the different sides of the sheet and is preferablytransparent on at least one side of the sheet. The cover sheet suitablycomprises a thermoplastic film, for example polyvinyl chloride,polyethylene terephthalate and polycarbonate compositions.

The cover sheet can be a supportive card-like sheet and if desired mayitself be a laminate suitably where a functional feature is to beretained between the layers of the laminate. Such sheets areparticularly suitable for stand-alone uses for example credit cards,security cards and card-keys where a suitable thickness may about 200 μmfor the cover sheets and 50 to 300, preferably 100 to 275 μm, for thereceiver sheet.

For security card applications, it is particularly desirable to providea finished card which conforms to the ISO standard thickness of 760μm±80 μm.

For other applications, much thinner cover sheets may be preferred forexample pouch laminates in which both cover sheets on a receiver sheetextend beyond the edge of the sheet and are bonded together around theirperiphery.

The dye-receiving layer preferably comprises at least one dye-receptivepolymer which is suitably an amorphous polyester.

The polymer may comprise other polymers for example polyvinyl chlorideand polyvinyl alcohol/polyvinyl chloride copolymer as desired.

Commercially available examples of suitable amorphous polyesters includeVITEL (RTM) PE200 (Goodyear) and VYLON (RTM) polyesters (Toyobo)especially grades 103 and 200. Different grades of polyester may bemixed to provide a suitable composition as desired.

If desired, the receiver layer may also comprise a release agent. Apreferred release agent is the thermoset reaction product of at leastone silicone having a plurality of hydroxyl groups per molecule and atleast one organic polyfunctional N-(alkoxymethyl) amine resin which isreactive with the hydroxyl groups under acid catalysed conditions.

Suitably, the back coat, if present, comprises a cross-linked polymerbinder and is provided to impart desirable properties to the receiversheet for example improved handling characteristics and reduced tendencyto retransfer the dye at low temperatures. If desired, the back coat mayhave a textured surface which may be imparted by a filler material or bythe polymer per se.

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1

A porous plastics material substrate available under the trade nameTESLIN (RTM), available from PPG Industry Inc, of thickness 255 μm wascoated with a receiver layer solution consisting of the followingcomposition:

    ______________________________________                                        VYLON 600                    100 g                                            (Polyester available from Toyobo)                                             BEETLE RTM) 692              6 g                                              (A poly functional N-alkyl Methyl amine cross-linking                         agent available from British Industrial Plastics)                             TEGOMER (RTm) 2311           0.075 g                                          (A hydroxy functional silicone                                                release agent available from Goldschmidt)                                     TIMUVIN (RTM) 900            1.0 g                                            (A hydroxylated benztriazole uv                                               absorber available from Ciba Geigy)                                           p-toluene sulpHonic acid     0.5 g                                            ______________________________________                                    

The receiver layer solution was a 20% solids solution inmethylethylketone: toluene (1:1) and was dried on the sub-layer at 80°C. for 2 minutes to give a receiver sheet according to the invention.

The receiver sheet was cut into 100×126 mm rectangles and givenregistration marks and were then printed in a thermal transfer printingprocess in a Hitachi VY200 video printer using a dye sheet availablefrom ICI Imagedata under catalogue number 105010 to provide cyan,magenta and yellow colour blocks of varying optical densities.

In this process, there were no mis-feeds, double-feeds ormis-registration of the receiver sheet.

The optical density of the colour blocks was determined using a MacbethTR1224 densitometer.

The maximum optical densities were as follows: Yellow--up to 2.72; Cyan--up to 2.30; Magenta--up to 2.67.

EXAMPLE 2

The imaged receiver sheets produced in Example 1 were laminated using ahot roll laminator Type 5020 (available from Morane Ltd) at atemperature of 170° C. on the receiver layer side with a cover sheet ofDDOT (a hot melt polyester adhesive coated transparent polyethyleneterephthalate film available from Transilwrap) and on the opposite side,a cover sheet of 7/3 (a hot melt ethylene/vinyl acetate adhesive coatedtransparent polyethylene terephthalate film available from Transilwrap)and cut into 2×10 cm strips.

The strips were then subjected to a peel test using an Instron 6021mechanical tester. The imaged side exhibited a bond strength of 30 to 40N/cm and the non imaged side, a bond strength of 20 to 30 N/cm.

The cards were found to be extremely difficult to delaminate by handalthough it was possible to delaminate strips which failed either in thesubstrate or the sub-layer thus providing evidence of tampering. Therewas no failure between the cover sheet and the imaged receiver layer.

EXAMPLE 3

An imaged receiver sheet produced in accordance with Example 1 and cutinto a flush cut card was immersed in water at 20° C. for 30 minutes andwas found to be structurally intact and was intact following a 40° C.wash cycle in an automatic washing machine thus demonstrating goodresistance to water.

A flush cut paper-based substrate receiver sheet absorbed water, swelledand the paper core delaminated demonstrating its lack of durability whenexposed to water.

EXAMPLE 4

A series of receiver sheet samples were produced in accordance withExample 1 except that prior to the application of the receiver layer thesubstrate samples were coated by reverse gravure with 20% solids aqueousemulsions to give a 1 μm sub-layer of the following materials:

Sample A Standard--no sub-layer

Sample B Comparative--VINAMUL (RTM) 3303, an ethylene/vinyl acetatecopolymer available from Vinamul Ltd.

Sample C VISCALEX (RTM) VG2, an acrylic acid/vinyl acetate copolymeravailable from Allied Colloids;

Sample D CARBOPOL (RTM) 907, an acrylic acid/vinyl acetate copolymeravailable from B F Goodrich;

Sample E TEXICOTE (RTM) 03052, a poly vinyl alcohol available from ScottBader;

Sample F DIOFAN (RTM) 185D, an acrylic acid/vinylidene chloridecopolymer available from BASF;

Sample G DIOFAN 193D, an acrylic acid/vinylidene copolymer availablefrom BASF;

Each sample was printed in a Hitachi VY200 printer at full power using amagenta dye sheet. The Optical Density of the resultant colour block wasmeasured using the Macbeth densitometer.

The samples were subjected to elevated temperatures and the opticaldensities re-measured. The results are shown in the Table.

Identical samples were prepared and laminated to a DDOT cover sheet andsubjected to a peel test as described in Example 2. The results areagain shown in the Table.

                  TABLE                                                           ______________________________________                                                        60 HOURS AT 80° C.                                     60 HOURS AT 60° C.                                                                       INI-   FI-         BOND                                     SAM-  INITIAL  FINAL   DIFF TIAL NAL  DIFF STRENGTH                           PLE   OD       OD      %    OD   OD   %    (N/cm)                             ______________________________________                                        A     1.96     1.18    40.0 1.90 0.54 71.0 <10                                B     1.94     1.26    34.6 1.91 0.67 65.0 >30                                C     1.95     1.84    5.6  1.87 1.7  9.1  10                                 D     1.95     1.83    6.1  1.89 1.68 11.1 10                                 E     1.96     1.84    6.1  1.88 1.66 11.7 10                                 F     1.95     1.93    1.0  1.92 1.68 12.5 >30                                G     1.94     1.91    1.5  1.93 1.64 15.0 >30                                ______________________________________                                    

EXAMPLE 5

Example 4 was repeated except that sub-layers were formed from VICLAN(RTM) 801, VICLAN 834 and VICLAN 872 (acrylic acid/vinylidene chloridecopolymers available from ICI). Similar results to Samples F and G wereobtained except that the VICLAN 872 had a lower bond strength of 25N/cm.

EXAMPLE 6

Sample B of Example 4 was repeated except that the sub-layer containedin addition 10% of EASTMAN (RTM) SIZE WD30 (a sulphonated polyesteravailable from Eastman Kodak). An improvement in the bond strength to25N/cm was achieved at the expense of an increase in the dyepermeability value at 80° C. to 15%.

EXAMPLE 7

Example 4 was repeated except that resin bonded paper (E86016 availablefrom Felix Schoeller) and plain paper were used as substrates. Similarresults were obtained.

We claim:
 1. A thermal transfer printing receiver sheet comprising asubstrate having on one side thereof a dye receiving layer,characterised in that the substrate comprises a porous plastics materialhaving a network of interconnecting pores communicating throughout thesubstrate.
 2. A receiver sheet according to claim 1 wherein said poresconstitute at least 50% by volume of said substrate.
 3. A receiver sheetaccording to claim 2, wherein said pores constitute 60 to 70% by volumeof said substrate.
 4. A receiver sheet according to claim 1, 2 or 3,wherein the pores have a maximum dimension of 0.01 to 20 μm.
 5. Areceiver sheet according to claims 1, 2 or 3, wherein a sub-layer isinterposed between the substrate and the dye receiving layer.
 6. Areceiver sheet according to claim 5, wherein the sub-layer is such as toprovide the receiver sheet with a dye permeability value at 60° C. ofless than 20%.
 7. A receiver sheet according to claim 5, wherein thesub-layer is such as to provide the receiver sheet with a dyepermeability value at 80° C. of less than 20%.
 8. A receiver sheetaccording to claim 5, wherein the sub-layer has a bond strength to thesubstrate of at least 10 N/cm.
 9. A receiver sheet according to claim 5,wherein the sub-layer has a solids content on coating of at least 20 wt%.
 10. A receiver sheet according to claim 5, wherein the material ofthe sub-layer is an acrylic acid/vinyl acetate copolymer, an acrylicacid/vinylidene chloride copolymer or a poly vinyl alcohol.
 11. Athermal transfer printing receiver sheet comprising a substrate havingon one side a dye receiving layer, wherein the substrate comprises aporous plastic material and a sub-layer is interposed between thesubstrate and the dye receiving layer, characterised by the sub-layerbeing such as to provide the receiver sheet with a dye permeabilityvalue at 60° C. of less than 20%.
 12. A receiver sheet according toclaim 11, wherein the sub-layer is such as to provide the receiver sheetwith a dye permeability value at 80° C. of less than 20%.
 13. A receiversheet according to claim 11 or 12, wherein the sub-layer has a bondstrength to the substrate of at least 10 N/cm.
 14. A receiver sheetaccording to any of claims 11 or 12, wherein the sub-layer has a solidscontent on coating of at least 20 wt %.
 15. A receiver sheet accordingto claim 11, wherein the material of the sub-layer is an acrylicacid/vinyl acetate copolymer, an acrylic acid/vinylidene chloridecopolymer or a poly vinyl alcohol.
 16. A receiver sheet according toclaim 11, wherein the substrate is a porous plastics material having anetwork of interconnecting pores communicating throughout the substrate.17. A receiver sheet according to claim 16, wherein said poresconstitute at least 50% by volume of said substrate.
 18. A receiversheet according to claim 16, wherein said pores constitute 60 to 70% byvolume of said substrate.
 19. A receiver sheet according to claim 16,17, or 18, wherein the pores have a maximum dimension of 0.01 to 20 μm.